thermokarst
n.— «Nearby were the outlines of other, even bigger holes, which, Romanovsky told me, had been filled with gravel by the local public-works department. The holes, known as thermokarsts, had appeared suddenly when the permafrost gave way, like a rotting floorboard.» —“The Climate Of Man” by Elizabeth Kolbert New Yorker Apr. 18, 2005. (source: Double-Tongued Dictionary)